This invention relates to the field of stepping motors and particularly to stepping motor drive circuits for achieving high resolution.
Development is stepping motors over the years has made them attractive to design engineers when slow or intermittent rotational motion is required. In conventional stepping motor applications, speed and direction of rotation is determined by the phase and frequency of power pulses applied to its stator windings with the angular rotation being directly proportional to the number of switching pulses applied thereto. Consequently, stepping motors are easily controlled by digital logic systems using standard integrated circuit logic and augmented by power amplifier circuits. Some commercially available stepping motors can be incremented in steps of 0.72.degree. which corresponds to 500 switching pulses being applied to the stator windings for each revolution of its rotor. Stepping motors having a 1.8.degree. step angle per pulse, however, are more readily available and are made in a wider range of sizes. Accordingly, stepping motors which provide 1 full revolution of the rotor for every 200 switching pulses are more commonly encountered in stepping motor applications.
For stepping motors of the type having 1.8.degree. rotation for each stepping pulse, the degree of positional precision is limited by the number of pulses which must be applied to the stator windings in order to complete one revolution of the rotor. As such, these motors by themselves are capable of coming to rest at only one of 200 unique positions. While this number of unique positions may be more than adequate for many applications of stepping motors, they are not well suited for positioning of diffraction gratings or the like in optical instruments because the incremental steps are larger than is desirable for such applications.
In overcoming the resolution limitations of stepping motors of the type described above, engineers have resorted to gearing systems or combinations of two or more stepping motors to provide greater positional resolution. These approaches frequently add considerably to instrumentation cost as well as mechanical complexity which, to a large measure, negates the value of using a stepping motor in the first place. Addtionally, where gearing systems are employed, the overall response time is significantly reduces as the resolution is increased and expensive antibacklash techniques must be employed.
A further problem with utilizing stepping motors of the type described above with conventional pulse controls arises from the fact that each time the motor is stepped, there is an accompanying tendancy to generate vibrations in the driven elements as well as in the support for the stepping motor. In some cases, this vibration is so severe as to limit the utility of the motor for a particular application. Some approaches have been developed to mitigate these vibrations which include gearing, as described above, or viscous damping. These approaches, however, are effective in reducing vibrations at the expense of decreased slew rate and increased stepping response time. A further approach for overcoming the vibration problem is to drive a stepping motor smoothly, without steps, by an electronic oscillator or a mechanical sine-cosine generator. System versitility, however, is lost because, in the first place, analogue sinusoidal oscillators have a finite minimum frequency, and in the second place, exclusively electronic control is precluded.
In view of the foregoing and other difficulties experienced in using stepping motors, it is an important objective of the present invention to provide a stepping motor control circuit capable of achieving higher resolution than has heretofore been achieved.
It is a further objective of the invention to provide a stepping motor control circuit which, while achieving high resolution, does not degrade the response time.
It it yet a further objective of the invention to provide a stepping motor control circuit which permits the assembly to be mechanically simple in construction while achieving increased resolution.
It is yet a further objective of the invention to provide a stepping motor control circuit capable of providing increased resolution while minimizing the cost for the control circuit.
It is still another objective of the invention to provide a high resolution stepping motor control that maintains position integrity.